Methods for treating the surfaces of aluminium alloys by means of formulations containing alkane sulfonic acid

ABSTRACT

Aluminum or aluminum alloys are surface-treated by anodic oxidation (anodization) in an electrolyte containing from 3 to 30% by weight of an alkanesulfonic acid. Workpieces based on aluminum or aluminum alloys and produced by this process can be used in building and construction, in automobile or aircraft construction and in the packaging industry.  
     An electrolyte composition containing from 3 to 30% by weight of an alkanesulfonic acid can be used in the anodic oxidation of aluminum or aluminum alloys (anodization) to increase the rate of anodic oxidation and to reduce the energy consumption.

[0001] The invention relates to a process for the surface treatment of aluminum or aluminum alloys by anodic oxidation of the aluminum or aluminum alloy (anodization) and to the use of an alkanesulfonic acid in a process for the anodic oxidation of aluminum or aluminum alloys, an electrolyte composition for the anodic oxidation of aluminum or aluminum alloys and the use of workpieces based on aluminum or aluminum alloys and produced by the process of the present invention.

[0002] In air, bare aluminum very quickly becomes covered with a very tin oxide skin which gives it a higher corrosion resistance than would be expected on the basis of its standard potential of −1.69 V. The corrosion resistance can be increased further by thickening the natural oxide skin by chemical or electrochemical methods. The thickened oxide skin is absorbent, so that it can be colored using water-soluble dyes or dye precursors. Furthermore, the oxide surfaces offer an excellent base for adhesion of paints and the abrasion resistance of workpieces is increased by anodic surface oxidation.

[0003] The surface oxidation of the aluminum surface or the surface of aluminum alloys can be carried out by electrochemical means by dipping the workpieces into solutions of slightly aggressive agents or by chromating and phosphating.

[0004] However, anodic oxidation by electrochemical means (anodization, eloxal process) is generally more advantageous, since thicker oxide coatings can be obtained in this way than by chemical treatment

[0005] The most frequently used processes employ sulfuric acid (S), oxalic acid (X) or chromic acid solutions as electrolyte. Exclusively direct current is used in the chromic acid process, while the sulfuric acid and oxalic acid processes are carried out using either direct current (DS or DX process) and i alternating current (AS or AX process). It is also possible to use a mixture of sulfuric acid and oxalic acid (DSX process). It is therefore of some relevance that the mixture can be used at higher bath temperatures (22-24° C.) than can an electrolyte based on pure sulfuric acid (18-22° C.). In these processes, the thickness of the oxide layer is from about 10 to 30 μm.

[0006] At low temperatures (up to about +10° C., preferably from 2 to 3° C.), high current densities (up to 2.5 A/dm²) and generally low sulfuric acid concentrations (up to about 10% strength by weight), if desired in admixture with phosphoric acid, very hard, abrasion-resistant oxide layers are obtained (bard anodizing). Here, a thickness of the oxide layer of >50 μm can be achieved. These workpieces obtained by hard anodization are used, in particular, for aluminum pressure castings, e.g. for engine construction. There is a maximum achievable layer thickness, which in the case of the DS process, for example, is about 45 μm. At this maximum layer thickness, the dissolution rate of the aluminum oxide is equal to its formation rate.

[0007] In addition, there are further specific anodic oxidation processes, e.g. aluminum coil coating (for can manufacture) which is generally carried out by passing an aluminum strip through a sulfuric acid electrolyte. Here, layer thicknesses of from 2 to 3 μm are desired.

[0008] It is an object of the present invention to provide an anodization process for aluminum or aluminum alloys which is faster than the classical processes of the prior art and also gives a better current yield, i.e. suffers from lower energy losses due to cooling. This process should be suitable both for anodization by dipping and for continuous anodization, e.g. of strip or wire by means of an electrolytic pull-through process. Furthermore, the process should, in hard anodization, make it possible to achieve a greater maximum layer thickness than is possible using the processes of the prior art, e.g. the DS process.

[0009] We have found that this object is achieved by a process for the surface treatment of aluminum or aluminum alloys by anodic oxidation of the aluminum or the aluminum alloys (anodization) in an electrolyte containing from 3 to 30% by weight of an alkanesulfonic acid.

[0010] The electrolyte preferably contains from 10 to 30% by weight, particularly preferably from 10 to 25% by weight, of an alkanesulfonic acid. In addition, the electrolyte may further comprise other acids, in particular acids selected from among sulfuric acid, phosphoric acid and oxalic acid. In a preferred embodiment, the electrolyte comprises sulfuric acid in addition to an alkanesulfonic acid. In a further preferred embodiment, an electrolyte based exclusively on an alkanesulfonic acid is used.

[0011] The use of alkanesulfonic acids in the surface treatment of aluminum or aluminum alloys is already known from the prior art. However, these known processes concern essentially the use of alkanesulfonic acids in the electrolytic metal salt coloring of aluminum, where an alkanesulfonic acid is used as additive or basis of an acid electrolyte solution, and not the use of alkanesulfonic acid in anodic oxidation (anodization) of aluminum or an aluminum alloy.

[0012] Thus, U.S. Pat. No. 4,128,460 relates to a process for coloring aluminum or aluminum alloys by electrolysis, comprising the anodization of aluminum or the aluminum alloys by customary methods and subsequent electrolysis in a bath comprising an aliphatic sulfonic acid and a metal salt, in particular a tin, copper, lead or silver salt, of the sulfonic acid. According to U.S. Pat. No. 4,128,460, the stability of the electrolysis bath is increased by an increased oxidation stability of the metal salts used and a uniform coloration of the surface of the aluminum or the aluminum alloys achieved.

[0013] The Brazilian patent applications BR 91001174, BR 9501255-9 and BR 9501280-0 also relate to processes for coloring the eloxidized aluminum by electrodipping, using electrolytes and metal salts which are mainly composed of pure methanesulfonic acid, methanesulfonates of tin or copper or methanesulfonates of nickels lead or other salts. According to these patent applications, an increase in the specific electrical conductivity of the solution, a reduction in the time for coloring in a simple manner and with reliable control, reproducibility of the color shade and low operating costs are achieved in this way.

[0014] Only BR 9501255-9 discloses specific reaction conditions for anodization of the surface of aluminum, with the use of methanesulfonic acid as additive in an electrolyte based on sulfuric acid being mentioned. In this electrolyte, methanesulfonic acid is used in an amount of 10 parts by weight based on sulfuric acid, i.e. less than 2% by weight of the electrolyte. No further indication of the use of alkanesulfonic acids in the anodization step or advantages of such a use are disclosed in BR 9501255-9.

[0015] According to the present invention, it has been found that use of alkanesulfonic acids as basis of the electrolytes used in the anodization step leads to more rapid anodization than when using the methods of the prior art. This is also of critical importance in respect of subsequent electrolytic coloration of the anodized surface, since the anodization is the rate-determining step in such a two-stage process comprising anodization and subsequent coloration of the anodized surface. The anodization step is, depending on the color of the surface, from 5 to 50 times slower than the subsequent coloration step. Increasing the rate of the anodization step thus makes the process more economical since higher throughputs per unit time can be achieved.

[0016] The electrolysis time for achieving an aluminum oxide layer thickness optimum for a subsequent coloration step, which is generally from 10 to 30 μm, preferably from 15 to 25 μm, is generally from 5 to 40 minutes, preferably from 10 to 30 minutes, with the precise time being dependent, inter alia, on the current density.

[0017] Furthermore, alkanesulfonic acids have a significantly lower corrosive action on the aluminum oxide layer formed in the anodization than does, for example, the sulfuric acid customarily employed. The process of the present invention thus makes it possible, particularly in hard anodization, to achieve greater layer thicknesses in a shorter time than when using the processes of the prior art.

[0018] A further great advantage of the process of the present invention is the significantly lower energy consumption during anodization, since a significantly lower voltage compared to the pure sulfuric acid electrolyte is established at the same current. As a consequence, the energy required for cooling the anodization bath is significantly lower.

[0019] The process of the present invention is suitable both for anodization of aluminum or aluminum alloys by the electrodipping process and for continuous anodization, for example of strip, pipe or wire, by means of an electrolytic pull-through process, e.g. for producing aluminum sheets for can manufacture.

[0020] The process of the present invention can be operated either using direct current or using alternating current; the process is preferably carried out using direct current.

[0021] In addition to the alkanesulfonic acid, the electrolyte can farther comprise other acids, for example sulfuric acid, phosphoric acid or oxalic acid. In a preferred embodiment of the process of the present invention, the electrolyte comprises either an alkanesulfonic acid or a mixture of sulfuric acid and alkanesulfonic acid as only acid. The electrolyte preferably comprises from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of a further acid selected from among sulfuric acid, phosphoric acid and oxalic acid, where the sum of alkanesulfonic acid and sulfuric acid, phosphoric acid or oxalic acid is 100 parts by weight and makes up from 3 to 30% by weight of the electrolyte. The electrolyte particularly preferably comprises from 20 to 90 parts by weight of an alkanesulfonic acid and from 80 to 10 parts by weight of sulfuric acid. The use of alkanesulfonic acid as sole acid in the electrolyte is, however, likewise possible.

[0022] For the purposes of the present invention, alkanesulfonic acids are aliphatic sulfonic acids. The aliphatic radical of these may, if desired, be substituted by functional groups or heteroatoms, e.g. hydroxy groups. Preference is given to using alkanesulfonic acids of the formulae.

R—SO₃H or HO—R′—SO₃H,

[0023] Here, R is a hydrocarbon radical which may be branched or unbranched and has from 1 to 12 carbon atoms, preferably 1 to 6 carbon atoms, particularly preferably an unbranched hydrocarbon radical having from 1 to 3 carbon atoms, very particularly preferably 1 carbon atom, i.e. methanesulfonic acid.

[0024] R′ is a hydrocarbon radical which may be branched or unbranched and ha from 2 to 12 carbon atoms, preferably 2 to 6 carbon atoms, particularly preferably an unbranched hydrocarbon radical having from 2 to 4 carbon atoms, where the hydroxy group and the sulfonic acid group can be bound to any carbon atoms, with the restriction that they are not bound to the same carbon atom.

[0025] According to the present invention, particular preference is given to using methanesulfonic acid as alkanesulfonic acid

[0026] Aluminum and aluminum alloys can be anodically oxidized by the process of the present invention, Particularly suitable aluminum alloys are alloys of aluminum with silicon, manganese, zinc, copper and/or magnesium. In these, silicon, manganese, zinc, copper and/or magnesium can be present in the alloy in a proportion of 15% by weight (Si), 4% by weight (Mn), 5% by weight (Zn), 5% by weight (Cu) and 5% by weight (Mg), with casting alloys also being included.

[0027] In the case of some aluminum materials, a tendency for pit corrosion to occur is found when using electrolytes comprising alkanesulfonic acids. In such cases, a brief preanodization step in sulfuric acid electrolytes is advantageous In the subsequent anodization in an alkanesulfonic acid electrolyte, the aluminum oxide skin which has already been formed protects the workpiece from corrosive attack. This preanodization step is generally carried out for a period of from 3 s to 5 min, preferably for from 1 to 3 minutes.

[0028] The present invention accordingly also provides a process in which the anodic oxidation is carried out in two stages, comprising:

[0029] preanodization of the aluminum or the aluminum alloy in an electrolyte comprising sulfuric acid as sole acid or a mixture of sulfuric acid and oxalic acid;

[0030] oxidation in an electrolyte according to the present invention comprising an alkanesulfonic acid.

[0031] The process conditions of the preanodization preferably correspond to the conditions of the classical DS (direct current sulfuric acid) or DSX (direct current sulfuric acid-oxalic acid) electrolysis known from the prior art.

[0032] The anodic oxidation (anodization) is preferably carried out at from 0 to 30° C. If excessively high temperatures are employed, irregular deposition of the oxide layer occurs, which is undesirable.

[0033] In general, hard anodization in which thick oxide layers having a low porosity and thus a high hardness and high protection of the aluminum surface are sought is carried out at low temperatures of generally from 0 to 5° C., preferably from 0 to 3° C. Owing to the fact that alkanesulfonic acids are less corrosive toward aluminum oxide than is pure sulfuric acid, high thicknesses of the oxide layer of >30 μm, preferably from 40 to 100 μm, particularly preferably from 50 to 80 μm, are possible by means of the process of the present invention in shorter times than when using pure sulfuric acid as basis of the electrolyte. These aluminum oxide surfaces obtained by hard anodization are generally not used for a subsequent step to color the surface.

[0034] The anodization according to the present invention for obtaining a porous aluminum oxide surface which is particularly well-suited for subsequent coloration of the surface is generally carried out at from 17 to 30° C., preferably from 18 to 28° C. The process of the present invention differs from processes of the prior art in that it can be carried out at a higher temperature than the processes of the prior art. Usually, temperatures above about 24° C. give unusable, nonuniform oxide layers, while the process of the present invention allows the anodization to be carried out at up to 30° C. The ability of the process to be carried out at higher temperatures saves energy costs. In general, cooling of the electrolyte solution during anodization is necessary, since the anodization is exothermic. This embodiment of the process of the present invention at generally from 17 to 30° C. gives, depending on the current density and the electrolysis time, layer thicknesses of from 5 to 40 μm, preferably from 10 to 30 μm.

[0035] The process of the present invention leads to aluminum oxide surfaces which are optimally suited to subsequent coloration, so that uniformly colored aluminum oxide layers can be obtained.

[0036] The process of the present invention is generally carried out at a current density of from 0.5 to 5 A/dm², preferably from 0.5 to 3 A/dm², particularly preferably from 1 to 2.5 A/dm². The voltage is generally from 1 to 30 V, preferably from 2 to 20 V.

[0037] Apart from the alkanesulfonic acid or mixture of alkanesulfonic acid and sulfuric acid used according to the present invention, the electrolyte generally further comprises water and, if necessary, further additives such as aluminum sulfate.

[0038] Apparatuses suitable for carrying out the process of the present invention are generally all known apparatuses which are suitable for electrodipping or for continuous anodic oxidation of aluminum or aluminum alloys, e.g. by means of an electrolytic pull-through process. Particular preference is given to using apparatuses made of metals which are resistant to alkanesulfonic acids or apparatuses which are lined with plastic, e.g. polyethylene or polyproylene.

[0039] The present invention further provides a process for the surface treatment of aluminum or aluminum alloys, comprising the following steps:

[0040] a) pretreatment of the aluminum or the aluminum alloy;

[0041] b) anodic oxidation by the process of the present invention (anodization);

[0042] c) if desired, coloration of the oxidized surface of the aluminum or the aluminum alloys;

[0043] d) after-treatment of the workpiece obtained after steps a), b) and, if employed, c);

[0044] e) if desired, recovery of the alkanesulfonic acid used and/or its salts, where step e) can follow or be carried out in parallel with any step in which an alkanesulfonic acid can be used, in particular the steps b) and/or, if employed, c).

[0045] Step a)

[0046] The pretreatment of the aluminum or the aluminum alloys is a critical step since it determines the optical quality of the end product. Since the oxide layer produced in anodization is transparent and this transparency is retained during the coloration process in step c), every surface defect on the metallic workpiece remains visible on the finished part.

[0047] The pretreatment is generally carried out by customary methods such as mechanical polishing or electropolishing, dewaxing using neutral surfactants or organic solvents, brightening or pickling. This is generally followed by rinsing with water.

[0048] In a preferred embodiment of the present invention, solutions comprising alkanesulfonic acids are preferably also used in step a) (e.g. in the case of brightening and electropolishing). Preferred alkanesulfonic acids have already been mentioned above for use in the anodizing step (step b)). Particular preference is given to using methanesulfonic acid.

[0049] Step b)

[0050] Step b) is the anodization process according to the present invention which follows the pretreatment of the aluminum or the aluminum alloy. This process according to the present invention has been described in detail above.

[0051] Step c)

[0052] If the anodized aluminum or the anodized aluminum alloy is not to be used directly without coloration of the aluminum oxide layer, which is generally the case for, for example, hard anodization, in which case dense, thick layers are obtained, the aluminum oxide layer obtained in step b) can be colored.

[0053] Coloration of the aluminum oxide layer occurs by uptake of organic or inorganic dyes into the capillary-shaped pores of the oxide layer obtained by anodization in step b).

[0054] For the purposes of the present invention, it is generally possible to use all processes known from the prior art for coloring anodized aluminum in step c). A distinction is usually made between chemical and electrolytic coloration

[0055] In chemical coloration, anodized aluminum or aluminum alloy is colored in the aqueous phase by means of suitable organic or inorganic compounds in the absence of an electric current. Organic dyes (eloxal dyes, e.g. dyes from the alizarin series or indigo dyes) often have the disadvantage of being insufficiently lightest. Inorganic dyes can, in a chemical coloration step, be deposited in the pores by precipitation reactions ox by hydrolysis of heavy metal salts. However, the processes which occur here are difficult to control and there are frequently reproducibility problems, i.e. problems in obtaining constant color shades For this reason, electrolytic processes for coloring aluminum oxide layers have become increasingly established for some time.

[0056] Step c) of the process of the present invention is therefore preferably carried out by an electrolytic method in an electrolyte comprising metal salts.

[0057] The aluminum oxide layers obtained after step b) of the process of the present invention are colored in an electrolyte comprising metal salts by means of direct or alternating current, preferably by means of alternating current. Here, metal is deposited in the bottom of the pores of the oxide layer from the metal salt solution The use of salts of various metals and various operating conditions give different colors. The colors obtained are very lightfast.

[0058] Suitable metal salts are generally salts selected from among tin, copper, silver, cobalt, nickel, bismuth, chromium, palladium and lead and mixtures of two or more of these metal salts. Preference is given to using tin, copper or silver salts or Yes thereof in the process of the present invention.

[0059] In general, the sulfates of the abovementioned metals are used and electrolyte solutions based on sulfuric acid are used. Additives can be additionally added to the electrolyte to improve the scatter and reduce oxidation of the metal ions used, e.g. the oxidation of tin(II) to the insoluble tin(IV).

[0060] In a particularly preferred embodiment of the process of the present invention, the electrolyte comprises from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of sulfuric acid, where the sum of alkanesulfonic acid and sulfuric acid is 100 parts by weight and makes up from 0.1 to 20% by weight, preferably from 0.1 to 15% by weight, of the electrolyte. The electrolyte very particularly preferably comprises 100 parts by weight of an alkanesulfonic acid

[0061] Alkanesulfonic acids suitable for step c) of the process have been disclosed above for use in the anodization (step b)). Particular preference is given to methanesulfonic acid.

[0062] Compared to purely sulfuric acid electrolytes, electrolytes based on alkanesulfonic acids have a higher electrical conductivity, bring about more rapid coloration and display a reduced oxidation action, as a result of which the precipitation of, for example, tin(IV) salts from electrolytes comprising tin(II) salts is prevented and the addition of additives such as environmentally harmful phenolsulfonic or toluenesulfonic acid is not necessary.

[0063] The metal salts are generally used in a concentration of from 0.1 to 50 g/l, preferably from 0.5 to 20 g/l, particularly preferably from 0.2 to 10 g/l, based on the metal used, in the electrolyte.

[0064] In addition to the appropriate acid, preferably sulfuric acid or an alkanesulfonic acid or a mixture of the two acids, and the metal salt used or a mixture of a plurality of metal salts, the electrolyte generally firer comprises water and, if necessary, further additives such as scattering improvers. However, particularly when using electrolytes comprising alkanesulfonic acids, the addition of additives is generally not necessary.

[0065] The electrolysis time in step c) is generally from 0.1 to 10 minutes, preferably from 0.5 to 8 minutes, particularly preferably from 0.5 to 5 minutes, with the electrolysis time depending on the metal salts used and the desired depth of color.

[0066] The electrolytic coloration in step c) is usually carried out using alternating current. The current density is generally from 0.1 to 2 A/dm², preferably from 0.2 to 1 A/dm². The voltage is generally from 3 to 30 V, preferably from 5 to 20 V.

[0067] All apparatuses suitable for the electrolytic coloration of aluminum oxide layers can be used.

[0068] Suitable electrodes are the electrodes which are usually suitable in a process for the electrolytic coloration of aluminum oxide layers, for example stainless steel or graphite electrodes. It is also possible to use one electrode made of the metal to be deposited, e.g. tin, silver or copper.

[0069] In a particularly preferred embodiment of the process of the present invention a gold color of the oxidized surface of the aluminum or the aluminum alloys is achieved in an electrolyte comprising silver salts, if desired in admixture with tin salts and/or copper salts. Such gold-colored aluminum workpieces are of particular interest for producing decorative objects, since the demand for gold-colored aluminum objects is great.

[0070] These gold-colored aluminum oxide surfaces are preferably obtained by carrying out the coloration process in step c) at a concentration of an alkanesulfonate of silver, calculated as Ag⁺, of from 2 to 50 g/l, preferably from 3 to 20 g/l, and a product of current density and voltage of from 0.5 to 10 AV/dm², preferably from 1 to 5 AV/dm², for a period of generally from 0.05 to 4 minutes, preferably from 0.3 to 3 minutes. A precise description of the production of gold-colored aluminum oxide layers may be found in the patent application DE-A . . . having the title “Production of gold-colored surfaces of aluminum or aluminum alloys by means of silver-con ng formulations”, which was filed at the same time.

[0071] Step d)

[0072] The after-treatment of the workpiece obtained after step b) or, if employed, c) may be divided into two steps:

[0073] d1) Rinsing

[0074] To remove residues of the bath from the pores of the oxide layer, the workpieces are generally rinsed with water, in particular with running water. This rinsing step follows both step b) and step c) if this is carried out.

[0075] d2) Sealing

[0076] Subsequent to step b), if step c) is not carried out, or subsequent to step c) if this is carried out, the pores of the oxide layer produced are generally sealed to provide good corrosion protection, This sealing can be achieved by dipping the workpieces into boiling distilled water for from about 30 to 60 minutes. This causes swelling of the oxide layer, as a result of which the pores are closed. The water can also contain additives. In a particular embodiment, the workpieces are after-treated in pressurized steam of from 4 to 6 bar instead of in boiling water.

[0077] Further methods of sealing are possible, for example by dipping the workpieces into a solution of readily hydrolyzable salts, as a result of which the pores are blocked by sparingly soluble metal salts, or into chromate solutions, which is predominantly employed for alloys rich in silicon and/or heavy metals. Treatment in dilute water glass solutions also leads to sealing of the pores if the silica is precipitated by subsequent dipping into sodium acetate solution. Furthermore, the pores can be sealed by means of insoluble meal silicates or organic, water-repellent substances such as waxes, resins, oils, paraffins, varnishes and plastics.

[0078] However, sealing is preferably carried out by means of water or steam.

[0079] e) Recovery of the Alkanesulfonic Acid Used and/or its Salts

[0080] To save costs and for ecological reasons, the alkanesulfonic acid used and/or its salts can be recovered. This recovery can follow or be carried out in parallel with any step in which an alkanesulfonic acid can be used, Recovery can be carried out, for example, in combination with the rinsing step (d1) following step b) and, if it is carried out, step c). Such a recovery can be carried out, for example, by means of electrolytic membrane cells, by cascade rinsing, or by simple concentration, for example, of the rinsing solutions.

[0081] The present invention further provides for the use of an alkanesulfonic acid in a process for the anodic oxidation of aluminum or aluminum alloys (anodization) for increasing the rate of the anodic oxidation This makes it possible to achieve more rapid aluminum oxide deposition than when using the processes of the prior art. Furthermore, in hard anodization, thicker layers can be obtained in a shorter time when using alkanesulfonic acids as basis of the electrolyte than when using pure sulfuric acid as electrolyte basis. In addition, the energy consumption is significantly lower since a lower voltage is established and less cooling has to be employed.

[0082] Furthermore, an electrolyte composition containing from 3 to 30% by weight of an alkanesulfonic acid for the anodic oxidation of alumina or aluminum alloys is claimed. Preference is given to an electrolyte composition comprising from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of sumac acid, where the sun of alkanesulfonic acid and sulfuric acid is 100 parts by weight and makes up from 3 to 30% by weight of the electrolyte. Suitable alkanesulfonic acids have already been mentioned above. The alkanesulfonic acid used is particularly preferably methanesulfonic acid. These electrolyte compositions are very suitable for use in a process for the anodic oxidation of aluminum or aluminum alloys and lead to more rapid aluminum oxide deposition than the processes of the prior art and to a thicker aluminum oxide layer in a shorter time, which is of particular interest in hard anodization, and to a reduced energy consumption.

[0083] The workpieces based on aluminum or aluminum alloys produced according to the present invention can be used, for example, in building and construction in particular for producing window profiles or exterior wall components, in automobile or aircraft construction, both for producing body parts and for producing aluminum pressure castings, e.g. in engine construction, and in the packaging industry, in particular for producing cans, for example by a continuous electrolytic pull-through process, e.g. continuous coil anodization

[0084] The following examples illustrate the invention.

EXAMPLES Example 1

[0085] Anodization electrolytes comprising, in each case, 18% by weight of an acid or an acid mixture and 8 g/l of aluminum were used. The electrolytes were used for the anodization of pure aluminum sheets which had in each case been preanodied for 2 minutes by the classical DS method. Anodization was in each case carried out at a current density of 1.2 A/dm² for 30 minutes. The anodization bath was in each case thermostated at 20° C. The thickness of the aluminum oxide layer, the porosity or microstructure of the surface and the microhardness were determined on the anodized workpieces. Table 1 below shows the thicknesses of the oxide layer obtained as a function of the electrolyte used and the anodization voltage and any cooling necessary: TABLE 1 Thickness of the Anodization Cooling Electrolyte oxide layer in μm voltage in V necessary 1.¹⁾ H₂SO₄ 12 ca. 12 Strong 2.¹⁾ H₂SO₄/oxalic acid 11 ca. 11 Strong (90:10) 3. MSA²⁾ 16 ca. 2.5 Slight 4. MSA/H₂SO₄ (50:50) 14 ca. 2.5 Slight

Example 2

[0086] This was carried out using a method analogous to Example 1, but electrolysis was carried out at 2° C. for 40 minutes.

[0087] The layers all displayed a significantly lower porosity and an increased hardness compared to Example 1. The aluminum sheets anodized in MSA (methanesulfonic acid) had a 20% greater thickness and an about 10% geater hardness than the aluminum sheets anodized in H₂SO₄.

Example 3

[0088] This was carried out by a method analogous to Example 1, but electrolysis was carried out at 28° C.

[0089] The layers all displayed a significantly increased porosity and a reduced hardness; the porosity of the aluminum sheets 3 and 4 (according to the present invention, the acid in the electrolyte corresponds to the compositions indicated in Table I under No.3 and 4, respectively) is lower than that of the others.

[0090] Coloring experiments in an electrolyte comprising silver methanesulfonate were carried out on all aluminum sheets. Only in the case of aluminum sheets 3 and 4 (experiments according to the present invention) were hi-quality gold colors achieved. In the case of aluminum sheet 2, relatively good gold colors were still achieved.

[0091] Coloration

[0092] A coloring electrolyte was made up from 19 g/l of silver methanesulfonate (10 g/l of Ag⁺) and 57 g/l of methanesulfonic acid. At a current density of 0.2 A/dm² and a voltage of about 8 V, the aluminum sheets anodized as indicated for No. 3 and 4 in Table 1 were colored for different periods of time. For both aluminum sheets, the colors indicated in Table 2 below were obtained, TABLE 2 Color at 0.2 Time [sec] A/dm² 15 Pale gold 30 Light gold 60 Gold 120 Gold 180 Deep gold 

We claim:
 1. A process for the surface treatment of aluminum or aluminum alloys by anodic oxidation of the aluminum or the aluminum alloys (anodization) using direct current in an electrolyte containing from 3 to 30% by weight of an alkanesulfonic acid.
 2. A process as claimed in claim 1, wherein the electrolyte comprises from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of a further acid selected from among sulfuric acid, phosphoric acid and oxalic acid, where the sum of alkanesulfonic acid and the further acid is 100 parts by weight and the concentration of the alkanesulfonic acid is from 3 to 30% by weight of the electrolyte.
 3. A process as claimed in claim 1 or 2, wherein the alkanesulfonic acid is methanesulfonic acid.
 4. A process as claimed in any of claims 1 to 3, wherein the anodic oxidation is carried out at from 0 to 30° C.
 5. A process as claimed in any of claims 1 to 4, wherein the anodic oxidation is carried out in two stages, comprising: preanodization of the aluminum or the aluminum alloy in an electrolyte comprising sulfuric acid as sole acid or a mixture of sulfuric acid and oxalic acid, oxidation in an electrolyte comprising an alkanesulfonic acid as claimed in any of claims 1 to
 3. 6. A process for the surface treatment of aluminum or aluminum alloys, comprising the following steps: a) pretreatment of the aluminum or the aluminum alloy; b) anodic oxidation by a process as claimed in claims 1 to 5 (anodization); c) if desired, coloration of the oxidized surface of the aluminum or the aluminum alloys; d) after-treatment of the workpiece obtained after steps a), b) and, if employed, c); e) if desired, recovery of the alkanesulfonic acid used and/or its salts, where step e) can follow or be carried out in parallel with any step in which an alkanesulfonic acid can be used, in particular the steps b) and/or, if employed, c).
 7. A process as claimed in claim 6, wherein solutions comprising alkanesulfonic acids are also used in the pretreatment of the aluminum or the aluminum alloys in step a).
 8. A process as claimed in claim 6 or 7, wherein the coloration of the oxidized surface of the aluminum or the aluminum alloys in step c) is carried out by an electrolytic process in an electrolyte comprising metal salts.
 9. A process as claimed in claim 8, wherein a gold color of the oxidized surface of the aluminum or the aluminum alloys is achieved in an electrolyte comprising silver salts, if desired in admixture with tin salts and/or copper salts.
 10. A process as claimed in claim 8 or 9, wherein the electrolyte comprising metal salts comprises from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of sulfuric acid, where the sum of alkanesulfonic acid and sulfuric acid is 100% by weight and makes up from 0.1 to 20% by weight of the electrolyte.
 11. The use of an alkanesulfonic acid in a process for the anodic oxidation of aluminum or aluminum alloys (anodization) using direct current to increase the rate of anodic oxidation and to reduce the energy consumption
 12. An electrolyte composition consisting of from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of a further acid selected from among sulfuric acid, phosphoric acid and oxalic acid, where the sum of alkanesulfonic acid and further acid is 100 parts by weight and the concentration of the alkanesulfonic acid is from 3 to 30% by weight of the electrolyte, water and optionally further additives such as aluminum sulfate.
 13. An electrolyte composition as claimed in claim 12, wherein the alkanesulfonic acid is methanesulfonic acid.
 14. The use of workpieces with a surface based on aluminum or aluminum alloys, wherein the surface was treated by a process as claimed in any of claims 1 to 10, in building and construction, in particular for producing window profiles or components of exterior walls, in automobile or aircraft construction and in the packaging industry, in particular for producing cans. 